The differential phase contrast imaging or in short phase contrast imaging represents an imaging method which has received a lot of attention over the last few years particularly in the Talbot Lau interferometer arrangement. The publication by F. Pfeiffer et al., “Hard X-ray dark-field imaging using a grating interferometer”, Nature Materials 7, pages 134 to 137 describes that with the aid of an interferometric structure, which consists of a conventional x-ray tube, three gratings and an x-ray detector, both absorption contrast, differential phase contrast and also dark-field contrast can be reconstructed from the same data record. The same can also be inferred from Joseph J. Zambelli, et al., “Radiation dose efficiency comparison between differential phase contrast CT and conventional absorption CT”, Med. Phys. 37 (2010), pages 2473 to 2479, or Bech et al., “Soft-tissue phase-contrast tomography with an x-ray tube source”, Phys. Med. Biol. 54 (2009), pages 2747 to 2753, or Bech et al, “Quantitative x-ray dark-field computed tomography”, Physics in Medicine an Biology, 55:5529-5539, 2010.
The wave nature of particles such as x-ray quanta allows for the description of phenomena such as refraction and reflection with the aid of the complex refractive indexn=1−δ+iβ. 
Here the imaginary part β describes the absorption, which forms the basis of current clinical x-ray imaging, such as for instance computed tomography, angiography, radiography, fluoroscopy or mammography, and the real part δ describes the phase displacement which is observed during the differential phase contrast imaging.
DE 10 2010 018 715 A1 discloses an x-ray recording system, in which an x-ray recording system is used for phase contrast imaging of an examination object for the purpose of high-quality x-ray imaging, said x-ray recording system comprising at least one x-ray emitter with a plurality of field emission x-ray sources for emitting coherent x-rays, an x-ray detector, a diffraction grating G1 arranged between the examination object and the x-ray image detector and a further grating G2 which is arranged between the diffraction grating G1 and the x-ray image detector.
In the arrangements currently the focus of attention for clinical phase contrast imaging, conventional x-ray tubes, currently available x-ray image detectors, such as described for instance by M. Spahn in “Flat detectors and their clinical applications”, European Radiology, Volume 15 (2005), pages 1934 to 1947, and three gratings G0, G1 and G2 are used, such as is subsequently explained in more detail with the aid of FIG. 1, which indicates a schematic structure of a Talbot Lau interferometer for the differential phase contrast imaging with extended tube focus, gratings G0, G1 and G2 and a pixelated x-ray image detector.
The x-rays 12 originating from a tube focus 11 of a non-coherent x-ray emitter 32 penetrate an absorption grating (G0) in order to generate coherent radiation, said absorption grating effecting the local coherence of the x-ray source, and an examination object 6, for instance a human or animal patient. The wave front of the x-rays 12 through the examination object 6 is deflected by phase displacement such that, such as the normal 15 of the wave front without phase displacement, i.e. without object, and the normal 16 of the wave front with phase displacement indicate. The phase-displaced wave front then passes through a diffraction or phase grating 17 (G1) with a grating constant adjusted to the typical energy of the x-ray spectrum in order to generate interference lines and/or an interference pattern 18 and in turn an absorbing analyzer grating 19 (G2) for reading out the generated interference pattern 18. Different interference patterns 18 develop with and without an object. The grating constant of the analyzer grating 19 is adjusted to that of the phase grating 17 and the remaining geometry of the arrangement. The analyzer grating 19 is for instance arranged at the first or n'th Talbot distance (order). The analyzer grating 19 in this way converts the interference pattern 18 into an intensity pattern, which can be measured by a detector or x-ray image detector 4. Typical grating constants for clinical applications are in the order of a few μm, as is also inferred for instance from the cited citations.
If the x-ray source is sufficiently coherent, i.e. the tube focus 11 of the x-ray source is sufficiently small and the generated x-ray power is consequently sufficiently large, it is possible to dispense with the first grating G0, the absorption grating 13.
The differential phase displacement is now determined for each pixel of the x-ray image detector 4 according to the prior art such that by means of a so-called “phase stepping” 20, which is indicated by an arrow, the analyzer grating 19 (G2) is displaced in a number of steps (k=1, . . . K, with e.g. K=4 to 8) by a corresponding fraction of the grating constant at right angles to the beam direction of the x-rays 12 and laterally with respect to the arrangement of the grating structure and the signal Sk produced for this configuration during the recording is measured in the pixel of the x-ray image detector 4 and the produced interference pattern 18 is thus scanned. For each pixel, the parameters of a function describing the modulation (e.g. sinus function) are then determined by a suitable fit method, an adjustment or compensation method, on the thus measured signals Sk. These parameters are usually the amplitude A, the phase Φ and the average intensity I.
Three different images can then be generated from the comparison of certain derived variables from these fit parameters for each pixel once with and once without an examination object, i.e. patient:                absorption image,        differential phase contrast image (DPC) and        dark-field image.        
In other words, with dark-field images, the local, i.e. within a pixel, destruction of the coherence of the x-rays is imaged. According to current knowledge scatter centers below the actual system resolution contribute significantly to this effect. With the grating-based phase contrast imaging, an absorption contrast, phase contrast and dark-field image are simultaneously obtained.
The visibility, i.e. the standardized difference from the maximum and minimum signal (or more precisely: amplitude standardized to the average signal), is here a measure of the characterization of the quality of a Talbot Lau interferometer. It is defined as a contrast of the scanned modulation
  V  =                              I          max                -                  I          min                                      I          max                +                  I          min                      =                  A                  I          _                    .      
Where an image is mentioned below, the triumvirate of absorption, DPC and dark-field image is meant where applicable.
With clinical interventions, auxiliary objects or medical instruments are inter alia introduced into the human body during surgery or orthopedics. In conventional medical x-ray based imaging, these medical instruments, such as for instance guide wires, stents, catheters etc., in the further sense also contrast agent, are made visible with the aid of the absorption contrast, i.e. x-rays are more significantly absorbed on these objects than in the remaining body, thereby indicating a signal difference on a locally resolved detector. The position of these objects is to be controlled in many instances as easily as possible and with a low x-ray dose. In order to be able to display these objects with a good contrast, they are usually manufactured from materials which contain elements with a high harmonic order, such as for instance metals. In order to further increase the visibility of these objects, they are often provided with additional materials with a high absorption. Markers comprising a platinum-iridium alloy are thus applied for instance to guide wires or medical plastics for catheters are also enriched with barium sulphate. A medical adhesive, for instance onyx, is mentioned as a further example, which is mixed with fine tantalum powder, in order to render the same visible in the x-ray image.
As described previously, another method of rendering visible structures in the human body with the aid of x-rays relates to the phase contrast imaging and/or x-ray dark-field images, which primarily do not use the absorbing effect of the material on x-rays, but instead the effect of the phase displacement when passing through the object and/or the refraction of the x-rays when transmitting refractive index gradients. If the medical instruments proven in use with classical x-ray technology are used with a phase contrast imaging, the achieved results, in particular with respect to the display quality of the obtained x-ray images, are often inadequate.